The speed ratio of a belt type continuously variable transmission is determined by a speed change control pressure supplied to a pulley cylinder chamber from a speed change control valve, and the speed change control pressure is adjusted by rotating a step motor to stroke the speed change control valve in an axial direction. Furthermore, by making a drive frequency of the step motor variable, the shifting speed can be varied.
JP10-225189A published by the Japan Patent Office describes a technique of controlling a shifting speed by making the drive frequency of a step motor variable in accordance with an oil temperature. At extremely low temperatures, the viscosity of the oil increases, and as a result, the driving force required to drive a spool of the speed change control valve cannot be obtained in the step motor, causing the motor to fall out of step. To prevent this, the drive frequency of the step motor is limited to a low frequency such that the shifting speed decreases. A technique whereby the drive frequency is limited to a low frequency at high temperatures to prevent oil leakage, which causes reductions in the amount of oil, is also known.